Assessment and modelling of the activation-dependent shift in optimal length of the human soleus muscle in vivo.

Previous in vitro and in situ studies have reported a shift in optimal muscle fibre length for force generation (L0) towards longer length at decreasing activation levels (also referred to as length-dependent activation), yet the relevance for in vivo human muscle contractions with a variable activation pattern remains largely unclear. By a combination of dynamometry, ultrasound and electromyography (EMG), we experimentally obtained muscle force-fascicle length curves of the human soleus at 100%, 60% and 30% EMGmax levels from 15 participants aiming to investigate activation-dependent shifts in L0 in vivo. The results showed a significant increase in L0 of 6.5 ± 6.0% from 100% to 60% EMGmax and of 9.1 ± 7.2% from 100% to 30% EMGmax (both P < 0.001), respectively, providing evidence of a moderate in vivo activation dependence of the soleus force-length relationship. Based on the experimental results, an approximation model of an activation-dependent force-length relationship was defined for each individual separately and for the collective data of all participants, both with sufficiently high accuracy (R2 of 0.899 ± 0.056 and R2 = 0.858). This individual approximation approach and the general approximation model outcome are freely accessible and may be used to integrate activation-dependent shifts in L0 in experimental and musculoskeletal modelling studies to improve muscle force predictions. KEY POINTS: The phenomenon of the activation-dependent shift in optimal muscle fibre length for force generation (length-dependent activation) is poorly understood for human muscle in vivo dynamic contractions. We experimentally observed a moderate shift in optimal fascicle length towards longer length at decreasing electromyographic activity levels for the human soleus muscle in vivo. Based on the experimental results, we developed a freely accessible approximation model that allows the consideration of activation-dependent shifts in optimal length in future experimental and musculoskeletal modelling studies to improve muscle force predictions.

Spatiotemporal modulation of a common set of muscle synergies during unpredictable and predictable gait perturbations in older adults.

Muscle synergies as functional low-dimensional building blocks of the neuromotor system regulate the activation patterns of muscle groups in a modular structure during locomotion. The purpose of the current study was to explore how older adults organize locomotor muscle synergies to counteract unpredictable and predictable gait perturbations during the perturbed steps and the recovery steps. Sixty-three healthy older adults (71.2±5.2 years) participated in the study. Mediolateral and anteroposterior unpredictable and predictable perturbations during walking were introduced using a treadmill. Muscle synergies were extracted from the electromyographic activity of 13 lower limb muscles using Gaussian non-negative matrix factorization. The four basic synergies responsible for unperturbed walking (weight acceptance, propulsion, early swing and late swing) were preserved in all applied gait perturbations, yet their temporal recruitment and muscle contribution in each synergy were modified (P<0.05). These modifications were observed for up to four recovery steps and were more pronounced (P<0.05) following unpredictable perturbations. The recruitment of the four basic walking synergies in the perturbed and recovery gait cycles indicates a robust neuromotor control of locomotion by using activation patterns of a few and well-known muscle synergies with specific adjustments within the synergies. The selection of pre-existing muscle synergies while adjusting the time of their recruitment during challenging locomotor conditions may improve the effectiveness to deal with perturbations and promote the transfer of adaptation between different kinds of perturbations.

Acceptability of Two Perturbation-Based Balance Training Paradigms: Perturbation Treadmill versus Dynamic Stability Training in the Presence of Perturbations.

INTRODUCTION: Perturbation-based balance training (PBT) is promising for fall prevention in older adults, mimicking real-life fall situations at a person's stability thresholds to improve reactive balance. Hence, it can be associated with anxiety, but knowledge about the acceptability of PBT is scarce. METHOD: This is a secondary analysis of a randomized controlled trial comparing the effects of two different PBT paradigms that aims to evaluate and compare the acceptability of those training paradigms in fall-prone older adults. Participants (74.9 ± 5.7 years) who completed the training (6 weeks, 3x/week) on either a perturbation treadmill (PBTtreadmill: n = 22) or unstable surfaces in the presence of perturbations (PBTstability: n = 27) were surveyed on the acceptability of PBT using a 21-item questionnaire addressing seven domains (perceived effectiveness, tailoring, demand, safety, burden, devices, affective attitude), based on the theoretical framework of acceptability and context-specific factors. Relative scores (% of absolute maximum) for single items and domains were calculated. RESULTS: Median domain scores of perceived effectiveness, tailoring, safety, devices, and affective attitude were all ≥70% for both paradigms. The highest scores were obtained for tailoring (both paradigms = 100% [interquartile range 80-100%]). Domain scores of demand and burden were in the medium range (40-45%) for both paradigms. No significant differences between paradigms were found for any domain score. Two single items of safety differed significantly, with PBTtreadmill perceived as needing less support (p = 0.015) and leading less often to balance loss (p = 0.026) than PBTstability. CONCLUSION: PBT conducted on a perturbation treadmill or unstable surfaces is well accepted in this fall-prone older sample, even though it is conducted at individual stability thresholds. Tailoring may play a key role in achieving high levels of perceived effectiveness, appropriate levels of demand and burden, and a high sense of safety. PBT delivered on treadmills might be more appropriate for more anxious persons.

Evidence of different sensitivity of muscle and tendon to mechano-metabolic stimuli.

This study aimed to examine the temporal dynamics of muscle-tendon adaptation and whether differences between their sensitivity to mechano-metabolic stimuli would lead to non-uniform changes within the triceps surae (TS) muscle-tendon unit (MTU). Twelve young adults completed a 12-week training intervention of unilateral isometric cyclic plantarflexion contractions at 80% of maximal voluntary contraction until failure to induce a high TS activity and hence metabolic stress. Each participant trained one limb at a short (plantarflexed position, 115°: PF) and the other at a long (dorsiflexed position, 85°: DF) MTU length to vary the mechanical load. MTU mechanical, morphological, and material properties were assessed biweekly via simultaneous ultrasonography-dynamometry and magnetic resonance imaging. Our hypothesis that tendon would be more sensitive to the operating magnitude of tendon strain but less to metabolic stress exercise was confirmed as tendon stiffness, Young's modulus, and tendon size were only increased in the DF condition following the intervention. The PF leg demonstrated a continuous increment in maximal AT strain (i.e., higher mechanical demand) over time along with lack of adaptation in its biomechanical properties. The premise that skeletal muscle adapts at a higher rate than tendon and does not require high mechanical load to hypertrophy or increase its force potential during exercise was verified as the adaptive changes in morphological and mechanical properties of the muscle did not differ between DF and PF. Such differences in muscle-tendon sensitivity to mechano-metabolic stimuli may temporarily increase MTU imbalances that could have implications for the risk of tendon overuse injury.

Personalized tendon loading reduces muscle-tendon imbalances in male adolescent elite athletes.

An imbalanced adaptation of muscle strength and tendon stiffness in response to training may increase tendon strain (i.e., the mechanical demand on the tendon) and consequently tendon injury risk. This study investigated if personalized tendon loading inducing tendon strain within the effective range for adaptation (4.5%-6.5%) can reduce musculotendinous imbalances in male adolescent handball athletes (15-16 years). At four measurement time points during a competitive season, we assessed knee extensor muscle strength and patellar tendon mechanical properties using dynamometry and ultrasonography and estimated the tendon's structural integrity with a peak spatial frequency (PSF) analysis of proximal tendon ultrasound scans. A control group (n = 13) followed their usual training routine, an intervention group (n = 13) integrated tendon exercises into their training (3x/week for ~31 weeks) with a personalized intensity corresponding to an average of ~6.2% tendon strain. We found a significant time by group interaction (p < 0.005) for knee extensor muscle strength and normalized patellar tendon stiffness with significant increases over time only in the intervention group (p < 0.001). There were no group differences or time-dependent changes in patellar tendon strain during maximum voluntary contractions or PSF. At the individual level, the intervention group demonstrated lower fluctuations of maximum patellar tendon strain during the season (p = 0.005) and a descriptively lower frequency of athletes with high-level tendon strain (≥9%). The findings suggest that the personalized tendon loading program reduced muscle-tendon imbalances in male adolescent athletes, which may provide new opportunities for tendon injury prevention.

Addressing muscle-tendon imbalances in adult male athletes with personalized exercise prescription based on tendon strain.

PURPOSE: Imbalances of muscle strength and tendon stiffness can increase the operating strain of tendons and risk of injury. Here, we used a new approach to identify muscle-tendon imbalances and personalize exercise prescription based on tendon strain during maximum voluntary contractions (εmax) to mitigate musculotendinous imbalances in male adult volleyball athletes. METHODS: Four times over a season, we measured knee extensor strength and patellar tendon mechanical properties using dynamometry and ultrasonography. Tendon micromorphology was evaluated through an ultrasound peak spatial frequency (PSF) analysis. While a control group (n = 12) continued their regular training, an intervention group (n = 10) performed exercises (3 × /week) with personalized loads to elicit tendon strains that promote tendon adaptation (i.e., 4.5-6.5%). RESULTS: Based on a linear mixed model, εmax increased significantly in the control group over the 9 months of observation (pCon = 0.010), while there was no systematic change in the intervention group (pInt = 0.575). The model residuals of εmax, as a measure of imbalances in muscle-tendon adaptation, demonstrated a significant reduction over time exclusively in the intervention group (pInt = 0.007). While knee extensor muscle strength increased in both groups by ~ 8% (pCon < 0.001, pInt = 0.064), only the intervention group showed a trend toward increased normalized tendon stiffness (pCon = 0.824, pInt = 0.051). PSF values did not change significantly in either group (p > 0.05). CONCLUSION: These results suggest that personalized exercise prescription can reduce muscle-tendon imbalances in athletes and could provide new opportunities for tendon injury prevention.

Speed-specific optimal contractile conditions of the human soleus muscle from slow to maximum running speed.

The soleus is the main muscle for propulsion during human running but its operating behavior across the spectrum of physiological running speeds is currently unknown. This study experimentally investigated the soleus muscle activation patterns and contractile conditions for force generation, power production and efficient work production (i.e. force-length potential, force-velocity potential, power-velocity potential and enthalpy efficiency) at seven running speeds (3.0 m s-1 to individual maximum). During submaximal running (3.0-6.0 m s-1), the soleus fascicles shortened close to optimal length and at a velocity close to the efficiency maximum, two contractile conditions for economical work production. At higher running speeds (7.0 m s-1 to maximum), the soleus muscle fascicles still operated near optimum length, yet the fascicle shortening velocity increased and shifted towards the optimum for mechanical power production with a simultaneous increase in muscle activation, providing evidence for three cumulative mechanisms to enhance mechanical power production. Using the experimentally determined force-length-velocity potentials and muscle activation as inputs in a Hill-type muscle model, a reduction in maximum soleus muscle force at speeds ≥7.0 m s-1 and a continuous increase in maximum mechanical power with speed were predicted. The reduction in soleus maximum force was associated with a reduced force-velocity potential. The increase in maximum power was explained by an enhancement of muscle activation and contractile conditions until 7.0 m s-1, but mainly by increased muscle activation at high to maximal running speed.

Comparing the Effects of Two Perturbation-Based Balance Training Paradigms in Fall-Prone Older Adults: A Randomized Controlled Trial.

INTRODUCTION: There is increasing evidence that perturbation-based balance training (PBT) is highly effective in preventing falls at older age. Different PBT paradigms have been presented so far, yet a systematic comparison of PBT approaches with respect to feasibility and effectiveness is missing. Two different paradigms of PBT seem to be promising for clinical implementation: (1) technology-supported training on a perturbation treadmill (PBTtreadmill); (2) training of dynamic stability mechanisms in the presence of perturbations induced by unstable surfaces (PBTstability). This study aimed to compare both program's feasibility and effectiveness in fall-prone older adults. METHODS: In this three-armed randomized controlled trial, seventy-one older adults (74.9 ± 6.0 years) with a verified fall risk were randomly assigned into three groups: PBTtreadmill on a motorized treadmill, PBTstability using unstable conditions such as balance pads, and a passive control group (CG). In both intervention groups, participants conducted a 6-week intervention with 3 sessions per week. Effects were assessed in fall risk (Brief-BEST), balance ability (Stepping Threshold Test, center of pressure, limits of stability), leg strength capacity, functional performance (Timed Up and Go Test, Chair-Stand), gait (preferred walking speed), and fear of falling (Short FES-I). RESULTS: Fifty-one participants completed the study. Training adherence was 91% for PBTtreadmill and 87% for PBTstability, while no severe adverse events occurred. An analysis of covariance with an intention-to-treat approach revealed statistically significant group effects in favor of PBTstability in the Brief-BEST (p = 0.009, η2 = 0.131) and the limits of stability (p = 0.020, η2 = 0.110) and in favor of PBTtreadmill in the Stepping Threshold Test (p < 0.001, η2 = 0.395). The other outcomes demonstrated no significant group effects. CONCLUSION: Both training paradigms demonstrated high feasibility and were effective in improving specific motor performances in the fall-prone population and these effects were task specific. PBTtreadmill showed higher improvements in reactive balance, which might have been promoted by the unpredictable nature of the included perturbations and the similarity to the tested surface perturbation paradigm. PBTstability showed more wide-ranging effects on balance ability. Consequently, both paradigms improved fall risk-associated measures. The advantages of both formats should be evaluated in light of individual needs and preferences. Larger studies are needed to investigate the effects of these paradigms on real-life fall rates.

Effect of sex on muscle-tendon imbalances and tendon micromorphology in adolescent athletes-A longitudinal consideration.

Imbalances between muscle strength and tendon stiffness may cause high-level tendon strain during maximum effort muscle contractions and lead to tendon structural impairments and an increased risk for tendinopathy in adolescent athletes. However, it remains unclear whether the development of musculotendinous imbalances is influenced by sex. At four measurement time points during a competitive season, we measured quadriceps femoris muscle strength and patellar tendon mechanical properties in 15 female (14.3 ± 0.7 years) and 13 male (16.0 ± 0.6 years) elite handball players of similar maturity using dynamometry and ultrasonography. To estimate the tendon's structural integrity, the peak spatial frequency (PSF) of proximal tendon ultrasound scans was determined. Females demonstrated significantly lower muscle strength (p < 0.001) and patellar tendon stiffness (p < 0.001) than males with no significant changes over time (p > 0.05). Tendon strain during isometric maximum voluntary contractions and PSF neither differed between sexes nor changed significantly over time (p > 0.05). We found lower fluctuations in muscle strength (p < 0.001) in females during the season but no differences in the fluctuations of tendon strain, stiffness, and PSF (p > 0.05). Descriptively, there was a similar frequency (~40%) of athletes with high-level tendon strain (>9%) in both sexes. These findings suggest that the lower strength capacity of female athletes is paralleled by lower tendon stiffness. Thereby, muscle-tendon imbalances occur to a similar extent in both sexes leading to increased strain levels during the season, which indicates the need for specific tendon training.

Measuring Kinematic Response to Perturbed Locomotion in Young Adults.

Daily life activities often require humans to perform locomotion in challenging scenarios. In this context, this study aimed at investigating the effects induced by anterior-posterior (AP) and medio-lateral (ML) perturbations on walking. Through this aim, the experimental protocol involved 12 participants who performed three tasks on a treadmill consisting of one unperturbed and two perturbed walking tests. Inertial measurement units were used to gather lower limb kinematics. Parameters related to joint angles, as the range of motion (ROM) and its variability (CoV), as well as the inter-joint coordination in terms of continuous relative phase (CRP) were computed. The AP perturbation seemed to be more challenging causing differences with respect to normal walking in both the variability of the ROM and the CRP amplitude and variability. As ML, only the ankle showed different behavior in terms of joint angle and CRP variability. In both tasks, a shortening of the stance was found. The findings should be considered when implementing perturbed rehabilitative protocols for falling reduction.

Automated Method for Tracking Human Muscle Architecture on Ultrasound Scans during Dynamic Tasks.

Existing approaches for automated tracking of fascicle length (FL) and pennation angle (PA) rely on the presence of a single, user-defined fascicle (feature tracking) or on the presence of a specific intensity pattern (feature detection) across all the recorded ultrasound images. These prerequisites are seldom met during large dynamic muscle movements or for deeper muscles that are difficult to image. Deep-learning approaches are not affected by these issues, but their applicability is restricted by their need for large, manually analyzed training data sets. To address these limitations, the present study proposes a novel approach that tracks changes in FL and PA based on the distortion pattern within the fascicle band. The results indicated a satisfactory level of agreement between manual and automated measurements made with the proposed method. When compared against feature tracking and feature detection methods, the proposed method achieved the lowest average root mean squared error for FL and the second lowest for PA. The strength of the proposed approach is that the quantification process does not require a training data set and it can take place even when it is not possible to track a single fascicle or observe a specific intensity pattern on the ultrasound recording.

Enthalpy efficiency of the soleus muscle contributes to improvements in running economy.

During human running, the soleus, as the main plantar flexor muscle, generates the majority of the mechanical work through active shortening. The fraction of chemical energy that is converted into muscular work (enthalpy efficiency) depends on the muscle shortening velocity. Here, we investigated the soleus muscle fascicle behaviour during running with respect to the enthalpy efficiency as a mechanism that could contribute to improvements in running economy after exercise-induced increases of plantar flexor strength and Achilles tendon (AT) stiffness. Using a controlled longitudinal study design (n = 23) featuring a specific 14-week muscle-tendon training, increases in muscle strength (10%) and tendon stiffness (31%) and reduced metabolic cost of running (4%) were found only in the intervention group (n = 13, p < 0.05). Following training, the soleus fascicles operated at higher enthalpy efficiency during the phase of muscle-tendon unit (MTU) lengthening (15%) and in average over stance (7%, p < 0.05). Thus, improvements in energetic cost following increases in plantar flexor strength and AT stiffness seem attributed to increased enthalpy efficiency of the operating soleus muscle. The results further imply that the soleus energy production in the first part of stance, when the MTU is lengthening, may be crucial for the overall metabolic energy cost of running.

Prevention of strain-induced impairments of patellar tendon micromorphology in adolescent athletes.

High-level patellar tendon strain may cause impairments of the tendon's micromorphological integrity in growing athletes and increase the risk for tendinopathy. This study investigated if an evidence-based tendon exercise intervention prevents high-level patellar tendon strain, impairments of micromorphology and pain in adolescent basketball players (male, 13-15 years). At three time points over a season (M1-3), tendon mechanical properties were measured using ultrasound and dynamometry, proximal tendon micromorphology with a spatial frequency analysis and pain and disability using VISA-P scores. The control group (CON, n = 19) followed the usual strength training plan, including sprint and change-of-direction drills. In the intervention group (INT, n = 14), three sessions per week with functional exercises were integrated into the training, providing repetitive high-magnitude tendon loading for at least 3 s per repetition. The frequency of high-level strain (ie, ≥9%) continuously decreased in INT, while tending to increase in CON since tendon force increased in both (p < 0.001), yet tendon stiffness only in INT (p = 0.004). In CON, tendon strain was inversely associated with tendon peak spatial frequency at all time points (p < 0.05), indicating impairments of tendon micromorphological integrity with higher strain, but not at M2 and M3 in INT. Descriptively, the fraction of asymptomatic athletes at baseline was similar in both groups (~70%) and increased to 100% in M3 in INT, while remaining unchanged in CON. We suggest that functional high-load tendon exercises could reduce the prevalence of high-level patellar tendon strain and associated impairments of its micromorphology in adolescent athletes, providing new opportunities for tendinopathy prevention.

Reliable and effective novel home-based training set-up for application of an evidence-based high-loading stimulus to improve triceps surae function.

High-loading interventions aiming for muscle-tendon adaptations were so far implemented in on-site facilities. To make this evidence-based stimulus more accessible, we developed an easy-to-use sling-based training set-up for home-based Achilles tendon and triceps surae muscle strength training and assessed its reliability and effectiveness in healthy men. To assess reliability (n=11), intra-class correlation (ICC) and root mean square (RMS) differences of isometric maximum voluntary contraction (MVC) of the plantar flexors were used. Effectiveness was tested in a controlled intervention trial (n=12), applying one-legged high-loading intervention for 3 months with our mobile set-up, while the contralateral/untrained leg served as control, and assessing plantar flexor MVC, drop (DJ) and countermovement jump (CMJ) height. Reliability was excellent between (ICCB=0.935) and within session (ICCWs=0.940-0.967). The mean RMS difference between and within sessions was 5.3% and 4.7%, respectively. MVCs of the trained/intervention leg increased by 10.2±7% (P=0.004) (dynamometry) and 30.2±22.5% (mobile set-up) (P=0.012). MVC of the untrained/control leg did not change (P>0.05). DJ height increased (P=0.025; Dz=2.13) by 2.37±2.9cm. CMJ height (P>0.05) did not change. We recommend the evidence-based high-loading application with our novel home-based training set-up as reliable and effective improving strength and jump performance of the plantar flexor muscle-tendon unit.

A Simplified Method for Considering Achilles Tendon Curvature in the Assessment of Tendon Elongation.

The consideration of the Achilles tendon (AT) curvature is crucial for the precise determination of AT length and strain. We previously established an ultrasound-kinematic-based method to quantify the curvature, using a line of reflective foil skin markers covering the AT from origin to insertion. The current study aimed to simplify the method by reducing the number of markers while maintaining high accuracy. Eleven participants walked (1.4 m/s) and ran (2.5, 3.5 m/s) on a treadmill, and the AT curvature was quantified using reflective foil markers aligned with the AT between the origin on the gastrocnemius myotendinous-junction (tracked by ultrasound) and a marker on the calcaneal insertion. Foil markers were then systematically removed, and the introduced error on the assessment of AT length and strain was calculated. We found a significant main effect of marker number on the measurement error of AT length and strain (p<0.001). Using more than 30% of the full marker-set for walking and 50% for running, the R2 of the AT length error saturated, corresponding to average errors of <0.1 mm and <0.15% strain. Therefore, a substantially reduced marker-set, associated with a marginal error, can be recommended for considering the AT curvature in the determination of AT length and strain.

Effects of long-term athletic training on muscle morphology and tendon stiffness in preadolescence: association with jump performance.

PURPOSE: Evidence on training-induced muscle hypertrophy during preadolescence is limited and inconsistent. Possible associations of muscle strength and tendon stiffness with jumping performance are also not investigated. We investigated the thickness and pennation angle of the gastrocnemius medialis muscle (GM), as indicators for potential muscle hypertrophy in preadolescent athletes. Further, we examined the association of triceps surae muscle-tendon properties with jumping performance. METHODS: Eleven untrained children (9 years) and 21 similar-aged artistic gymnastic athletes participated in the study. Muscle thickness and pennation angle of the GM were measured at rest and muscle strength of the plantar flexors and Achilles tendon stiffness during maximum isometric contractions. Jumping height in squat (SJ) and countermovement jumps (CMJ) was examined using a force plate. We evaluated the influence of normalised muscle strength and tendon stiffness on jumping performance with a linear regression model. RESULTS: Muscle thickness and pennation angle did not differ significantly between athletes and non-athletes. In athletes, muscle strength was greater by 25% and jumping heights by 36% (SJ) and 43% (CMJ), but Achilles tendon stiffness did not differ between the two groups. The significant predictor for both jump heights was tendon stiffness in athletes and normalised muscle strength for the CMJ height in non-athletes. CONCLUSION: Long-term artistic gymnastics training during preadolescence seems to be associated with increased muscle strength and jumping performance but not with training-induced muscle hypertrophy or altered tendon stiffness in the plantar flexors. Athletes benefit more from tendon stiffness and non-athletes more from muscle strength for increased jumping performance.

Muscle Synergies in Parkinson's Disease.

Over the last two decades, experimental studies in humans and other vertebrates have increasingly used muscle synergy analysis as a computational tool to examine the physiological basis of motor control. The theoretical background of muscle synergies is based on the potential ability of the motor system to coordinate muscles groups as a single unit, thus reducing high-dimensional data to low-dimensional elements. Muscle synergy analysis may represent a new framework to examine the pathophysiological basis of specific motor symptoms in Parkinson's disease (PD), including balance and gait disorders that are often unresponsive to treatment. The precise mechanisms contributing to these motor symptoms in PD remain largely unknown. A better understanding of the pathophysiology of balance and gait disorders in PD is necessary to develop new therapeutic strategies. This narrative review discusses muscle synergies in the evaluation of motor symptoms in PD. We first discuss the theoretical background and computational methods for muscle synergy extraction from physiological data. We then critically examine studies assessing muscle synergies in PD during different motor tasks including balance, gait and upper limb movements. Finally, we speculate about the prospects and challenges of muscle synergy analysis in order to promote future research protocols in PD.

Modular organization of murine locomotor pattern in the presence and absence of sensory feedback from muscle spindles.

KEY POINTS: Locomotion on land and in water requires the coordination of a great number of muscle activations and joint movements. Constant feedback about the position of own body parts in relation to the surrounding environment and the body itself (proprioception) is required to maintain stability and avoid failure. The central nervous system may follow a modular type of organization by controlling muscles in orchestrated groups (muscle synergies) rather than individually. We used this concept on genetically modified mice lacking muscle spindles, one of the two main classes of proprioceptors. We provide evidence that proprioceptive feedback is required by the central nervous system to accurately tune the modular organization of locomotion. ABSTRACT: For exploiting terrestrial and aquatic locomotion, vertebrates must build their locomotor patterns based on an enormous amount of variables. The great number of muscles and joints, together with the constant need for sensory feedback information (e.g. proprioception), make the task of controlling movement a problem with overabundant degrees of freedom. It is widely accepted that the central nervous system may simplify the creation and control of movement by generating activation patterns common to muscle groups, rather than specific to individual muscles. These activation patterns, called muscle synergies, describe the modular organization of movement. We extracted synergies through electromyography from the hind limb muscle activities of wild-type and genetically modified mice lacking sensory feedback from muscle spindles. Muscle spindle-deficient mice underwent a modification of the temporal structure (motor primitives) of muscle synergies that resulted in diminished functionality during walking. In addition, both the temporal and spatial (motor modules) components of synergies were severely affected when external perturbations were introduced or when animals were immersed in water. These findings show that sensory feedback from group Ia/II muscle spindles regulates motor function in normal and perturbed walking. Moreover, when group Ib Golgi tendon organ feedback is lacking due to enhanced buoyancy, the modular organization of swimming is almost completely compromised.

The force-length-velocity potential of the human soleus muscle is related to the energetic cost of running.

According to the force-length-velocity relationships, the muscle force potential is determined by the operating length and velocity, which affects the energetic cost of contraction. During running, the human soleus muscle produces mechanical work through active shortening and provides the majority of propulsion. The trade-off between work production and alterations of the force-length and force-velocity potentials (i.e. fraction of maximum force according to the force-length-velocity curves) might mediate the energetic cost of running. By mapping the operating length and velocity of the soleus fascicles onto the experimentally assessed force-length and force-velocity curves, we investigated the association between the energetic cost and the force-length-velocity potentials during running. The fascicles operated close to optimal length (0.90 ± 0.10 L0) with moderate velocity (0.118 ± 0.039 Vmax [maximum shortening velocity]) and, thus, with a force-length potential of 0.92 ± 0.07 and a force-velocity potential of 0.63 ± 0.09. The overall force-length-velocity potential was inversely related (r = -0.52, p = 0.02) to the energetic cost, mainly determined by a reduced shortening velocity. Lower shortening velocity was largely explained (p < 0.001, R2 = 0.928) by greater tendon gearing, shorter Achilles tendon lever arm, greater muscle belly gearing and smaller ankle angle velocity. Here, we provide the first experimental evidence that lower shortening velocities of the soleus muscle improve running economy.

Trunk muscle strength and lumbo-pelvic kinematics in adolescent athletes: Effects of age and sex.

Considering their potential relevance for low-back pain, we investigated trunk muscle strength, sagittal lumbo-pelvic alignment while standing and lumbo-pelvic ratio during trunk flexion in adolescent athletes with regard to the effects of age and sex. Twenty-two early adolescent (EA: 13-15 years, 10 females) and 28 late adolescent (LA: 16-19 years, 14 females) high-level athletes (training duration more than 12 hours per week) participated in the study. We measured trunk extension and trunk flexion moments during maximum voluntary isometric contractions using a dynamometer. Further, we examined lumbo-pelvic kinematics in the upright standing position and during forward trunk bending using two 3-dimensional accelerometers. Using a lineal regression model in which the flexion moment from each participant was used as predictor for the corresponding extension moment, we found higher residuals (P < 0.001) in the EA compared to LA, indicating greater imbalances in the trunk muscle strength in EA. We found a higher lordosis in the upright position, greater pelvic rotation, and greater lordotic posture during the forward bending in females (P < 0.01). These age-related imbalances and sex-related characteristics in lumbo-pelvic kinematics might affect the neuromuscular control of trunk stability and the magnitude of spine loading. We recommend the implementation of specific coordination and stabilization programs for muscle groups that contribute to lumbo-pelvic kinematics and training routines that support a balanced strength development within the trunk muscles in adolescent athletes.